There are presently a number of methods for storing and retrieving digital data. For example, digital data may be stored on magnetic disks, magnetic tape, in optical systems or in semiconductor memories. Each of these storage devices has at least two internal states which can be used to represent the binary values of the digital information. Before the data can be stored in the device, it must first be encoded using unique combinations of two or more of these internal states.
For example, digital data may be stored on magnetic tape by using the direction of alignment of magnetic domains on the tape to represent digital data values. One common method of aligning the magnetic domains is to use align the domains perpendicular to the magnetic head which "writes" the data on the tape. A binary value of "0" is represented by aligning the north pole of the magnetic domain either facing toward the write head or away from the write head. A binary "1" is represented by aligning the domain in the opposite direction. When domains aligned in this fashion pass under a reading head in well-known fashion, the reading circuitry produces output voltages of different values which then can be interpreted as binary "1"s or "0"s.
It is also possible to record data on magnetic tape using transitions between a domain in one direction and a domain in the other to represent data values rather than the absolute direction of the domain alignment. This method of encoding is called phase encoding. The domain transitions on the magnetic tape result in transitions in the output signal produced by the read heads from one voltage level to another voltage level. An output signal transition in one direction, for example, from high voltage to low voltage, is used to represent a binary "0". A transition in the other direction (low voltage to high voltage) is used to represent a binary "1". With such a transition recording scheme when digital data with successive data bits having the same binary values such as "1111" or "0000" is recorded on the tape, an extra transition, called a "phase transition" must be located half way between the data transitions to allow the signal to return to the proper level, either high or low, so that the next succeeding data transition can occur in the proper direction.
One advantage of phase encoding is that it is "self-clocking". More specifically, it is necessary for the circuitry which reads the data from the magnetic tape to synchronize approximately to the data rate of the stored data so that the reading circuitry can examine the magnetic tape at the proper locations to obtain the data transitions. In some encoding schemes, a separate clock track is used. The clock track contains clock signals which are recorded at locations on the tape corresponding to the location of the data signals. The clock signals can be read and used by the reading circuitry to develop a time period or "window" during which the reading circuitry examines the tape for a corresponding data level transition.
In a phase encoding arrangement, the data transitions occur with sufficient regularity that a clock signal can be derived directly from the data in a well-known manner by using a phased-lock loop, matched filter circuitry or other well-known apparatus. There are two well-known problems which interfere with the derivation of a clocking signal from phase encoded data using prior art circuitry by causing a change in the apparent frequency of the recorded data.
One of these problems is "bit shifting". Theoretically, the data transitions should occur at either of two frequencies, depending on whether there are phase transitions present or not. However, when certain data patterns are recorded, due to well-known magnetic properties of magnetic recording media and the characteristics of reproduction of a digital signal from such media, the locations of the data transitions will often shift from their theorectical locations. It is possible for this shift to occur so rapidly that the reading circuitry, using the derived clocking signal, will not have sufficient time to adjust the clocking signal to synchronize with the shifted data. When this happens an error or data "dropout" occurs.
In addition, to bit shift, variations in the mechanical speed of the tape transport may cause a variation in the apparent frequency of the recorded data. Therefore, some mechanism must be used to change the derived clock frequency to avoid read errors.
Accordingly, prior art systems have been developed in order to compensate the derived clocking rate for bit shift and speed variations of the tape tranport. In particular, although the normal phase-locked loop circuitry which is used to derive the clock signals from the recorded data can compensate for the slow changes caused by speed variations in the magnetic tape transport, bit shifting is often so severe that the loop becomes unsynchronized. In order to avoid this problem, prior art devices have utilized an averaging arrangement in which the correction factor which is used to reset the phase-locked loop is derived from an average of the phase differences over a predetermined number of preceding bit times. Since bit shift usually affects only one or two bits and does not affect the overall data rate, this averaging technique is effective to remove the abberations caused by bit shift if an average is taken over a sufficiently long time to cause the bit shift errors to cancel out. Unfortunately, if averaging is done over a sufficient time to eliminate bit shift, the phase-locked loop may not be able to properly track speed variations in the magnetic tape transport.
Still other prior art circuits (of which U.S. Pat. No. 3,827,078 is an example) have attempted to remove the effects of bit shifting by detecting the phase error between the raw data recovered from the recording medium and the clock signals generated by the phase-locked loop and dynamically adjusting the width of the data recovery window in response to the magnitude of the error. However, this circuitry, while compensating for bit shift cannot compensate for speed variation errors.
It is therefore an object of the present invention to provide a data storage and retrieval system which has fewer data recovery errors than prior systems.
It is another object of the present invention to provide a data storage and retrieval system which can compensate for data frequency variations in the storage system.
It is yet another object of the present invention to provide a data retreival system which can compensate for bit shifts occurring in the storage system.
It is a further object of the present invention to provide a data retreival system in which data frequency variations on each track of multitrack magnetic tape can be accomodated independently.